1. Field of the Invention
The present invention relates to an oxygen sensing apparatus which operates according to the principle of an oxygen concentration cell, and an oxygen sensing method using this apparatus. More particularly, this invention is concerned with such an oxygen sensing apparatus that is capable of detecting a wide range of oxygen concentration of the measurement gas with high accuracy and/or improved operating response, and such a detecting method using this apparatus.
2. Discussion of the Related Art
As an apparatus for detecting an oxygen concentration of combustion exhaust gases such as those emitted by internal combustion engines of automobiles or various industrial furnaces to determine an air/fuel (A/F) ratio of an air-fuel mixture supplied to the engines or industrial furnaces, there is known an oxygen sensing apparatus which operates according to the principle of an oxygen concentration cell. The oxygen sensing apparatus has an electrochemical cell which consists of an oxygen-ion conductive solid electrolyte body, a reference electrode exposed to a reference gas, and a measuring electrode exposed to a measurement gas to be measured by the apparatus. In operation, an electromotive force is induced between the reference electrode and the measuring electrode, due to a difference in the oxygen concentration between the reference gas and the measurement gas, according to the principle of an oxygen concentration cell.
While the atmosphere may be used as the reference gas for the oxygen sensing apparatus of the above type, the apparatus may produce by itself a reference gas having a relatively high concentration of oxygen, by applying a comparatively small amount of current between the reference electrode and the measuring electrode, to thereby cause the electrochemical cell to effect an oxygen pumping action for supplying a reference-gas chamber with the reference gas.
In the oxygen sensing apparatus of the above type adapted to produce the reference gas by itself, however, the electromotive force (the output of the apparatus) generated by the electrochemical cell is remarkably increased as the oxygen concentration of the measurement gas is lowered, whereby the electromotive force no longer accurately reflects or represents the oxygen concentration of the measurement gas, causing a considerably large measuring error in the output of the apparatus. This may be because the surface of the measuring electrode suffers from lack or insufficiency of oxygen due to the oxygen pumping action of the electrochemical cell when the oxygen concentration of the measurement gas is low.
In this regard, it is considerably difficult to compensate or rectify the apparatus output to remove the measuring error which occurs upon measurement of low-oxygen-concentration measurement gases since the amount of such measuring error greatly varies depending upon the oxygen concentration of the gas. Accordingly, the measurement gas having a relatively low concentration of oxygen cannot be accurately measured by the oxygen sensing apparatus of the above known type wherein the reference gas is produced by its oxygen pumping action.
The measuring error as described above can be effectively reduced by reducing the current applied between the reference electrode and measuring electrode, so as to permit accurate measurement of oxygen concentration in a relatively low range. However, the reduction of the current results in considerable deterioration of the operating response of the sensing apparatus with respect to the measurement gas having a relatively high concentration of oxygen.